Open Data is data that are freely available for reuse/secondary data analysis. The “openness” of data can be viewed and treated differently based on a spectrum of openness ranging from being highly restricted (as in the case of personal health information) to being freely available data that are usable. In general, open data follows the FAIR principles of being: Findable, Accessible, Interoperable and Reusable.
Data.gov is a site of open data hosted by the U.S. Federal Government.
This website provides a number of tools for exploring, publishing, and sharing public datasets: http://opendata-tools.org/en/